I'm not 100% sure, but we had such a point, identified differently on a 2 engined Boeing. It was the point, where if you had a depressurisation at CRZ FL you could make an emergency descent to 10,000' within the specified pax 02 duration time, considering the MSA. This had an effect on easterly routes in the region of the high mountains of Asia. The routing had to be adjusted to allow this, or increased pax 02 had to be fitted. THis was a planning operation and in the end we did not fly the route. If this is the correct answer, and you are on a 4 engine a/c, I wonder at its relevance.
Regarding fuel after a descent you should be able to insert new cruise FL in the FMC and read calculated arrival fuel at diversion or destination. Either that or good old fashioned time v fuel flow.